1. Field of the Invention
The present invention relates to an arc tube of discharge lamp and a method of manufacturing an arc tube.
2. Related Art
FIG. 7 shows a discharge lamp of related art. A front end portion of an arc tube 5 is supported by a single lead support 2 that protrudes forward of an insulating base 1, and a rear end portion of the arc tube 5 is supported by a recess part 1a of a base 1, and further a rear-end-side portion of the arc tube 5 is gripped by a metallic support member S fixed to a front surface of the insulating base 1.
A front-end-side lead wire 8 that is led out from the arc tube 5 is fixed to the lead support 2 by welding, while a rear-end-side lead wire 8 is fixed, through a bottom wall 1b that forms the recess part 1a of the base 1, to a terminal 3 provided for the bottom wall 1b by welding. Reference character G is an ultraviolet shielding globe having the nearly cylindrical shape, which cuts ultraviolet component in a wavelength area that is harmful to the human body, of light emitted from the arc tube 5. This globe G is integrally welded to the arc tube 5.
The arc tube 5 is structured, as shown in FIG. 8, that a closed glass bulb 5a in which electrode rods 6, 6 are oppositely arranged and a light emitting substance (mercury or metal halogen) is enclosed, is formed between a pair of front and back pinch seal portions 5b, 5b. Into the pinch seal portions 5b, electrode assemblies A, A′ are sealed thereby to secure air tightness in the closed glass bulb 5a. The electrode assembly is formed by integrally joining the tungsten electrode rod 6 that protrudes into the closed glass bulb 5a and a molybdenum lead wire 8 that leads out from the pinch seal portion 5b through a molybdenum foil 7.
As a method of manufacturing this arc tube (mercury enclosing arc tube) 5, firstly, as shown in FIG. 9(a), from a lower opening end side of a cylindrical glass tube W in which a glass bulb w2 is formed midway of a linear extension portion w1, the electrode assembly A formed by integrally joining the electrode rod 6, the molybdenum foil 7 and the lead wire 8 is inserted. Then, a position q1 near the chamber portion w2 is primarily pinch-sealed. Next, as shown in FIG. 9(b), through a mercury supply nozzle N inserted into the glass tube W from an upper opening end side, mercury is supplied into the glass bulb w2. Next, as shown in FIG. 9(c), into the glass bulb w2, pellet P of the light emitting substance is put. Next, as shown in FIG. 9(d), another electrode assembly A′ having a bending part 8a at the lead wire 8 is inserted into the glass tube W and held by itself. Namely, the bending part 8a that is formed at the lead wire 8 and has the width larger than the inner diameter of the glass tube W comes into pressure-contact with the inner surface of the glass tube W, and by this pressure-contact power, the electrode assembly A′ inserted into the glass tube W is held in the inserted position by itself. Next, the opening end of the glass tube W is temporarily sealed using a burner. Further, the electrode assembly A′ inserted portion of the glass tube W is secondarily pinch-sealed, the temporarily sealed portion of the glass tube W is cut at the predetermined position, and the lead wire 8 is led out from the glass tube W.
It has been known that this kind of arc tube 5 has a problem of a phenomenon in which light flickers during lighting the arc tube (hereinafter, this phenomenon is referred to as flicker).
Mechanism of generation of this flicker is represented by the following reaction expressions:4ScI3+3SiO2→2Sc2O3+3SiI4  (1)nW+SiI4→SiWn+2I2  (2)4ScI3+3ThO2→2Sc2O3+3ThI4  (3)This mechanism can be explained as follows.
Namely, as shown in the expression (1), vitreous silica (SiO2) constituting the tube wall of the arc tube reacts with ScI3, so that a devitrification phenomenon is produced. SiI4 (Si in SiI4) produced at this time, as shown in the expression (2), reacts with the tungsten electrode, so that low melting metal (SiWn) is produced. Further, in a thoriadoped tungsten electrode, as shown in the expression (3), thoria (ThO2) disappears, the distance between the electrodes widens due to deformation and damage of the electrode, the restriking voltage increases, and a ballast becomes uncontrollable state, so that flicker occurs.
In the mechanism (reaction expressions) of occurrence of this flicker, in case that impure gas and water exist, the reaction is promoted more. Therefore, there have been proposals for preventing the occurrence of this flicker by lessening OH-group content in vitreous silica constituting the arc tube as disclosed in JP-A-11-329350, or by lessening water content in the enclosed substance (metal halide) in the closed glass bulb as disclosed in JP-A-2004-039323.
Further, in the closed glass bulb 5a of the conventional arc tube, mercury that performs buffer action is enclosed. The mercury is a harmful substance to environment. Correspondingly to social needs of reducing environmental pollution on the earth as much as possible, the development of a mercury-free arc tube that does not include mercury in the closed glass bulb 5a is being performed actively. A method of manufacturing this mercury-free arc tube, except for omission of the mercury supply step shown in FIG. 9(b) in the before-mentioned method of manufacturing the mercury including arc tube (refer to FIGS. 9(a) to 9(d)), is nearly the same as the method of manufacturing the mercury including arc tube.
However, it is insufficient for preventing the occurrence of flicker, by lessening the OH-group content in vitreous silica or lessening the water content in the pellet P of the enclosed substance (metal halide) as disclosed in JP-A-11-329350 and JP-A-2004-039323.